1. Field of the Invention
The present invention relates to a non mechanical modulating device for use in infrared detection and has particular, although not exclusive, use in thermal imaging cameras.
2. Discussion of Prior Art
In such cameras it is desirable to modulate the intensity, that is introduce a time variation of the intensity, of the incident beam of infrared radiation from the image scene, in particular in the 8-14 .mu.m wavelength region. Where the camera uses an array of pyroelectric detectors, the modulation is an essential part of the camera operation. For thermal imaging cameras based on other types of detectors, for example resistive bolometric detectors, capacitive bolometric detectors and ferroelectric detectors, the modulation may not be essential but is a preferred feature. The modulating device may have additional applications in other systems incorporating pyroelectric or other infrared detectors or detector arrays, for example gas sensing or measuring systems where the modulation of infrared radiation from a thermal source is required.
The present technique used to achieve this modulation in both thermal imaging and gas sensing systems makes use of a mechanical chopping device in the form of a rotating disc. Such devices consume substantial power which reduces the battery life when used on portable equipment. In addition, any non-uniformity in the emissivity of the disc can give rise to non-uniformity in the image and which leads to reduced camera performance.
There has been a long felt need for a more convenient, non-mechanical means of modulating the intensity of infrared radiation in thermal imaging cameras. However, the present non-mechanical methods of modulating infrared light in the spectral region of interest have proved to be unsatisfactory. Those based on the electro-optic or acoustic optic effects are bulky, costly and typically only function for one wavelength and one polarization. This is not effective for the wide band of wavelengths and random polarization encountered in a typical infrared image. These techniques normally only function satisfactorily with well collimated beams, in contrast to the low F-number optics used in thermal imaging systems. Attempts to use liquid crystal devices for modulation purposes have also met with very limited success.
Previously, weak modulation effects have been observed in the infrared region by using the injection of free carriers into semiconductor materials such as germanium (Ge) or silicon (Si) [Yamada, Elec. Lett. 19 No. 22 (922-944), McQuistan, J. App. Phys. 35 No. 4 (1243-1248)]. However, modulating devices for use in thermal imaging cameras have never reached practical operation in the 8-14 .mu.m spectral region. Similarly, devices relying on etalon effects in Si have been described for use in the far infrared but are not suitable for use with thermal imaging cameras as they operate over a narrow band of wavelengths outside the region of interest [H. Alius and G. Dodel, Infrared Phys. and Technol., Vol. 35, No. 1, pp 73-78 (1994), H. Alius and G. Dodel, Appl. Phys. Lett., 57 16 (1990), H. Alius and G. Dodel, Infrared Physics, 32 pp 1-11 (1981)].
Stronger modulation effects have been observed in the infrared region by using the injection of free carriers into germanium and utilising interband transitions between the split valence bands in germanium [Umeno et al, "High-Efficiency Ge Modulator for Infrared (Laser) Beams" Japanese Journal of Applied Physics, Supplements, vol. 40, 1971, Tokyo]. However, the device proposed provides fast modulation (typically 10-20 kHz) unsuitable for use with infrared imaging systems. For example, pyroelectric detector arrays typically operate with readout frequencies of between 50-150 Hz (i.e. around an order of magnitude slower). Furthermore, the device cannot provide the uniformity of modulation depth required for a modulator device for an imaging system.